The present invention relates generally to a pump unit including two plunger pumps connected in series and arranged in parallel, and more particularly to a brake control device using same for achieving restrained slippage and prevented lock of wheels of motor vehicles.
When the driving wheels undergo excessive driving torque due to acceleration, sudden change in road conditions, etc., a slippage may occur between the driving wheels and a road causing an ineffective transmission of engine torque to the road. Moreover, upon braking of a motor vehicle carried out suddenly or on a frozen road, the wheels undergo excessive braking force, and are thus apt to be locked. With the wheels locked, control of the vehicle is extremely difficult.
In order to restrain slip rotation (refer hereafter to as "acceleration slip") of the driving wheels and to prevent (refer hereafter to as "braking slip") the wheels from locking upon braking, JP-A 4-231255 proposes a braking device wherein a braking pressure is controlled with respect to a wheel cylinder of each wheel.
For obtaining quick restraint of acceleration slip of the driving wheels, etc., this braking device comprises a main pump and an auxiliary pump arranged in series, the former feeding brake fluid to the wheel cylinders of the driving and driven wheels, the latter being connected to a master cylinder. A brake-fluid return passage has one end connected to the wheel cylinders, and another end connected to a brake-fluid passage for fluid communication between the main pump and the auxiliary pump. A check valve is arranged to the return passage to prevent flow of brake fluid from the auxiliary pump to the wheel cylinders through the return passage. A reservoir tank is arranged to the return passage between the check valve and the wheel cylinders to temporarily accumulate brake fluid out of the wheel cylinders.
When acceleration slip occurs, the main pump and the auxiliary pump are actuated to feed brake fluid within the master cylinder to the wheel cylinders of the driving or driven wheels, thereby restraining excessive slippage of the driving wheels to achieve effective transmission of engine torque to the road.
An example of a pump unit including the above main pump and auxiliary pump arranged in series is seen, e.g. in JP 53-49305. This pump unit comprises two plunger pumps arranged in line along a camshaft, a discharge port of the upstream-side plunger pump being connected to a suction port of the downstream-side plunger pump so that fluid inhaled from a suction port of the upstream-side plunger pump with rotation of the camshaft is fed to the suction port of the downstream-side plunger pump through the discharge port of the upstream-side plunger pump, and fluid inhaled into the downstream-side plunger pump is discharged from the discharge port thereof.
However, the conventional pump unit as disclosed in JP 53-49305 has a drawback in that the suction and discharge ports of each plunger pump are arranged at ends thereof opposite to the camshaft makes the layout of the brake-fluid passage communicating with each suction and discharge port complicated. Moreover, the pump unit becomes extremely long in the plunger reciprocating direction, enlarging a casing thereof, resulting in not only deteriorated mounting performance to the vehicles, but increased weight and manufacturing cost.
It is, therefore, an object of the present invention to provide a pump unit with reduced size and weight. Another object of the present invention is to provide a brake control device using such pump unit.